This invention relates to plastic extrusion and, more particularly, to an apparatus for making test specimens which permit mechanical and other material properties of plastics used for making extruded products to be evaluated and methods relating to same.
Extrusion is a well known process for shaping plastic products such as rods, tubes, and other simple solid and hollow shapes. During extrusion, plastic material in a highly viscous molten state is forced through an orifice of a die. Plastics which can be extruded typically include thermoplastics like polyethylene, polypropylene, polystyrene, nylon, and polyvinyl chloride, to name a few.
The manufacture of the raw plastic material involves the precise blending of various chemicals under carefully controlled process conditions. Variations of these process steps in the plastic forming process can substantially affect the mechanical properties and performance of the formed plastic product. As a result, samples from batches of plastic are routinely tested to assess the performance of the plastics from each batch.
One method which is commonly used for assessing the mechanical properties and performance of a batch of plastic is to test the reaction of the plastic to specified impact forces. The plastics industry employs an impact test performed precisely in accordance with ASTM standard #256 Method A (Izod type). In particular, the test employs a rectangular plastic specimen having a length of 2.500 inches, a width of 0.500 inches and a predetermined thickness of 0.125 inches, 0.250 inches or 0.500 inches. A V-shaped notch is formed in one edge of the plastic at a location midway along the length of the specimen. The notch is cut to a specified depth of 0.100 inches +/xe2x88x920.002 inches, such that the width of the specimen below the notch is 0.400 inches +/xe2x88x920.002 inches. The specimen is then securely clamped in a vise such that the apex of the notch is precisely aligned with the top of the vise. The portion of the specimen extending above the top of the vise is then impacted by a specified weight gravitationally falling through a specified arc. The particular weight employed in these tests is selected in accordance with the anticipated characteristics of the plastics being tested. The impact of the weight on the specimen is measured in appropriate units (e.g. foot-pounds/inch), with the type of break being noted and recorded (e.g. no break, partial break, hinged break or complete break). The test results of both the break type and the corresponding impact are then analyzed to determine if the plastic performed to its specifications.
The above described impact test is often performed for each batch of plastic that will used to form the desired plastic product. ASTM standards recommend that the impact tests be performed on ten specimens, with average readings then being compared to specifications.
The test specimens are typically formed by some type of conventional plastic molding process. Unfortunately, test results from molded test specimens are not particularly useful in evaluating the mechanical properties and performance of materials used for making extruded plastic products because the mechanical properties and performance of the materials are significantly affected by the extrusion process. Process variables such as the extrusion machine, the extrusion process parameters, and the die design can significantly impact the properties of the material. Thus, molded test specimens do not provide accurate information pertaining to the material properties of the plastic as they relate to extruded products.
Accordingly, test specimens are needed which permit accurate evaluation of the mechanical and other material properties of plastics used in the manufacture of extruded products.
The invention is directed to an apparatus for making test specimens. The apparatus transforms a first length of a material, extruded through a die of an extruder in accordance with a predetermined set of extrusion parameters, into a second length of material useable as at least one test specimen for evaluating material properties of the material extruded according to the predetermined set of extrusion parameters. The apparatus comprises a plate having an orifice defining an input opening and an output opening, the input opening having a predetermined profile area that generally corresponds to a profile area of the orifice of the die, the input opening for receiving material extruded through the die orifice, the output opening having a predetermined profile area corresponding to that of the at least one test specimen.
The invention is further directed to a method of using the above apparatus for making test specimens. The method comprises aligning the orifice of the above transitioning plate apparatus with the orifice of the die, and using the transitioning plate apparatus to transform a first length of material, extruded through the die orifice by the extruder, in accordance with a predetermined set of extrusion parameters, into a second length of material having the desired test specimen profile area, the second length of material being useable as at least one test specimen for evaluating material properties of the material as extruded according to the predetermined set of extrusion parameters.